Efforts have been directed largely toward studies of glomerular capillary permeability to macromolecules, with emphasis on the influence of molecular charge and molecular shape or configuration. We have developed a fixed-charge model of the glomerular capillary wall, and tested this using previously available data on the filtration of dextran sulfate and diethylaminoethyl dextran in the rat. To do this, measurements of electrophoretic mobility as a function of molecular size were used to estimate the effective molecular charge of these dextran derivatives. The model was found to provide an excellent description of available data, assuming that the glomerular capillary wall has an effective fixed charge density of about 165 mEq/liter. Additional studies to examine the influence of molecular configuration were performed by comparing filtration rates in the rat glomerulus of neutral dextran and ficoll, a highly crosslinked copolymer of sucrose and epichlorohydrin. Intrinsic viscosity and sedimentation coefficient data indicate that ficoll is more nearly spherical and more compact than dextran, and we found its passage across the glomerular capillary wall to be restricted to a greater extent than that of dextran of comparable molecular radius. Effort was also devoted to the development of a mathematical model of bicarbonate reabsorption by the renal proximal tubule. This includes the effects of diffusion, convection, and chemical reaction within both tubule lumen and epithelial cells, and allows an examination of rate-controlling steps under various physiological conditions.